In a vehicle such as a hybrid vehicle having an engine and an electric motor as a drive source or electric vehicle, a battery mounted on a vehicle body is charged and driving force is generated using electric energy supplied by the battery. Generally, the battery-related power supply circuit is configured as a high-voltage circuit that handles high-voltage of 200 V or more, and the high-voltage circuit including the battery has an ungrounded configuration for ensuring safety, which is electrically insulated from the vehicle body that is the reference potential point of the ground.
In vehicles equipped with an ungrounded high-voltage battery, a ground fault detection device is provided in order to monitor an insulation state (ground fault) between a system provided with high-voltage battery, specifically a main power supply system leading from the high-voltage battery to a motor and the vehicle body. The ground fault detection device widely uses a method utilizing a capacitor called a flying capacitor.
FIG. 8 is a diagram showing a circuit example of a conventional ground fault detection device of the flying capacitor type. As shown in the figure, the ground fault detection device 400 is the device connected to the ungrounded high-voltage battery 300, and detects a ground fault of a system in which the high-voltage battery 300 is provided. Here, an insulation resistance between the positive electrode side and the ground of the high-voltage battery 300 is represented by RLp, and the insulation resistance between the negative electrode side and the ground thereof RLn. The combined resistance of the positive electrode side insulation resistance RLp and the negative electrode side insulation resistance RLn becomes an insulation resistance RL.
As shown in the figure, the ground fault detection device 400 is provided with a detection capacitor C1 operating as flying capacitor. In addition, in order to switch a measurement path, and control charging and discharging of capacitor C1, four switches S1 to S4 are provided around the detection capacitor C1. Furthermore, a switching element Sa for sampling a voltage for measurement corresponding to the charge voltage of the detection capacitor C1 is provided.
In the ground fault detection device 400, in order to calculate the insulation resistance RL, measurement operation is repeated as one cycle from V0 measurement period, Vc1n measurement period, V0 measurement period, to Vc1p measurement period. In any measurement periods, after charging the detection capacitor C1 with the voltage of the measurement target, the charging voltage of the capacitor C1 is measured. The detection capacitor C1 is discharged for the next measurement.
In the V0 measurement period, a voltage corresponding to the voltage of the high-voltage battery 300 is measured. For this reason, switching elements S1 and S2 are turned on, switching elements S3 and S4 are turned off, and the detection capacitor C1 is charged. That is, as shown in FIG. 9A, the high-voltage battery 300, a charge resistor R1, and the detection capacitor C1 becomes the measurement path.
At the time of measuring the charging voltage of the detection capacitor C1, as shown in FIG. 9B, the switching elements S1 and S2 are turned off while the switching elements S3 and S4 are turned on, the switching device Sa is turned on and sampling is performed by the control device 420. Thereafter, as shown in FIG. 9C, the switching element Sa is turned off and the detection capacitor C1 is discharged for the next measurement of the battery C1. When the charging voltage of the detection capacitor C1 is measured, the operation at the time of discharge of the detection capacitor C1 is the same in other measurement periods.
In the Vc1n measurement period, a voltage reflecting the influence of the negative electrode side insulation resistance RLn is measured. Thus, the switching elements S1 and S4 are turned on, the switching elements S2 and S3 are turned off, and the detection capacitor C1 is charged. That is, as shown in FIG. 10A, a path including the high-voltage battery 300, the charge resistor R1, the detection capacitor C1, the negative electrode ground side resistor R4, the ground, and the negative electrode side insulation resistance RLn becomes the measurement path.
In the Vc1p measurement period, a voltage reflecting the influence of the positive electrode side insulation resistance RLp is measured. Thus, the switching elements S2 and S3 are turned on, the switching elements S1 and S4 are turned off, and the detection capacitor C1 is charged. That is, as shown in FIG. 10B, a path including the high-voltage battery 300, the positive electrode side insulation resistance RLp, the ground, the positive electrode ground side resistor R3, the charge resistor R1, and the detection capacitor C1 becomes the measurement path.
It is known to obtain (RLp×RLn)/(RLp+RLn), based on (Vc1p+Vc1n)/V0 calculated from V0, Vc1n, and Vc1p obtained in these measurement periods. Therefore, the control device 420 in the ground fault detection device 400 calculates, by measuring V0, Vc1n and Vc1p, the insulation resistance RL that is the combined resistor of the positive electrode side insulation resistance RLp and the negative electrode side insulation resistance RLn. When the insulation resistance RL is less than the predetermined judgment reference level, it is judged that a ground fault has occurred, and an alarm is issued.